implicit branch: unify PorousMediaBox- and PorousMediaCC-Problem to ImplicitPorousMediaProblem. Delete porousmediaccproblem.hh. Deprecate PorousMediaBoxProblem in favor of ImplicitPorousMediaProblem. Adapt includes and names.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/branches/implicit@9745 2fb0f335-1f38-0410-981e-8018bf24f1b0